In general, lens sheets for use in projection screens, especially circular Fresnel lens sheets are formed by a pressing or casting method. However, these two production methods are long in production time, so that they have had the problems of low productivity and high production cost.
Japanese Patent Laid-Open Publication No. 9301/1991 describes, as a means for solving the aforementioned problems, a process for producing a lens sheet, comprising the steps of coating a first ionizing radiation curable resin onto the entire surface of a forming die in the shape of a flat plate; forming a resin bank of a second ionizing radiation curable resin on the first ionizing radiation curable resin; and superposing a laminating base material on the second ionizing radiation curable resin, and uniformly laying the second ionizing radiation curable resin over the first ionizing radiation curable resin with forcing out, by the use of a pressure roll, those air bubbles that enter between the base material and the forming die.
In the process described in the above Japanese Patent Laid-Open Publication No. 9301/1991, it is necessary to use, as the first ionizing radiation curable resin, a resin having a decreased viscosity prepared by diluting the second ionizing radiation curable resin; a resin having a decreased viscosity prepared by heating the second ionizing radiation curable resin to a high temperature; another resin having a low viscosity; or the like, in order to promote deaeration in the step that comes after the step of coating the first ionizing radiation curable resin.
However, if a solvent is used for diluting the second ionizing radiation curable resin, the problem of safety occurs, and it becomes necessary to add the drying step. Further, in the case where the second ionizing radiation curable resin is heated to a high temperature, it is necessary to install a heater or the like; and, in addition, such a problem occurs that resin components are changed as a result of the evaporation of monomer components contained in the resin. Furthermore, when another resin having a low viscosity, containing excessive monomers is used, a lens in the desired shape cannot be obtained due to cure shrinkage; and such a problem occurs that adjustment of refractive index is required between the first and second ionizing radiation curable resins.